The objective of the proposed research is to develop a novel non-invasive laser-based technology for transcranial functional imaging of the brain of small animals in vivo. Small animals are the preferred laboratory models for studying various diseases, and small animal imaging provides the opportunity to evaluate pathologic progression in a much-compressed time frame and with a much-improved resolution. By combining high optical contrast and diffraction-limited high acoustic resolution, the proposed technology, functional photoacoustic tomography (fPAT), can image the intact brain free of speckle artifacts. Besides structural information, the proposed fPAT can also provide functional information including blood volume and blood oxygenation. In the proposed fPAT technology, a short-pulsed laser beam penetrates into the tissue sample diffusively. The photoacoustic waves, due to thermoelastic expansion resulting from a transient temperature rise on the order of 10 mK caused by the laser irradiation, are then measured around the sample by wide-band ultrasonic transducers. The acquired photoacoustic waves are used to reconstruct, at ultrasonic resolution, the optical absorption distribution that reveals optical contrast. Optical contrast is sensitive to the molecular conformation of biological tissue and is related to certain physiological parameters such as the level of hemoglobin oxygenation. The proposed fPAT technology combines the high-contrast advantage of optical imaging with the high 3D resolution advantage of ultrasound imaging. The proposed technology does not depend on ballistic/quasi-ballistic or backscattered light as optical coherence tomography (OCT) does. Any light, including both singly and multiply scattered photons, contributes to the imaging signal; as a result, the imaging depth in fPAT is better than in OCT. The resolution is diffraction-limited by the detected photoacoustic waves rather than by optical diffusion; consequently, the resolution of fPAT is excellent (60 microns, adjustable with ultrasonic frequency). Furthermore, fPAT is free of the speckle artifacts present in OCT and pulse-echo ultrasonography, two analogous technologies. The proposed research will be accomplished by a comprehensive multi-disciplinary team comprised of members of the Department of Biomedical Engineering at Texas A&M University (overall system), the Department of Pathobiology at Texas A&M University (animal experiments), the University of Connecticut (ultrasound system), the NIH Resource Center on Medical Ultrasonic Transducer Technology at the University of Southern California (ultrasound hardware), the University of Michigan (ultrasound software), and the National Institutes of Health (comparative study with fMRI/PET). The exciting aspect of this project is that the Texas A&M group has already successfully achieved high-quality in vivo PAT images of the rat brain. Because blood vessels of the brain are clearly imaged, this technology provides a unique opportunity to assess the functional parameters in a given blood vessel with a pinpoint accuracy. The applicants propose to advance this technology and explore its potential applications with the following specific aims: (1) Develop an ultrasound array system for rapid data acquisition and a single-wavelength photoacoustic tomography (PAT) system; (2) Develop a functional PAT (fPAT) system using a dual-wavelength laser system; (3) Image small-animal brain tumors using fPAT; (4) Image small-animal brain traumas using fPAT; (5) Image small-animal brain activation using fPAT; (6) Image small-animal brain chemotherapy using fPAT; and (7) Compare fPAT with established imaging modalities including fMRI/PET.